One of the most common causes of chronic back pain is degenerative disc disease. The degeneration may start after a particular injury, or many occur due to multiple injuries over time. Degeneration usually takes several years. As the vertebrae grow closer, the openings in the back of the spine where the nerve roots leave the spinal canal become narrower. This can lead to pinching and irritation on the nerves, causing pain.
There are many surgical approaches and methods used to fuse the spine. Most involve the placement of a bone graft between the vertebrae. Supplemental hardware, such as plates, screws and cages may or may not be used, depending upon the indication.
An early cage design is described in U.S. Pat. No. 4,501,269 to Bagby, entitled “PROCESS FOR FUSING BONE JOINTS.” According to the method, a hole is bored transversely across the joint and a slightly larger cylindrical basket is driven into the hole, thereby spreading the bones in resistance to the tensile forces of the surrounding tissue. Immediate stabilization of the joint is achieved by the implantation of the rigid cylindrical basket. Subsequent bone-to-bone fusion is achieved, both through and about the basket, which is filled with bone fragments produced during the boring step.
The Bagby patent states that the process is applicable to any human or animal joint formed by opposed contiguous bony surfaces which are covered and separated by intervening cartilage and are surrounded by ligaments which resist expansion of the joint. Specific examples of such joints are a spinal joint between adjacent vertebrae or the ankle joint.
This stand-alone interbody fusion technique continued to evolve with material changes and the design of threaded cages to increase stability and decrease displacement rates. Bilateral, parallel implants were designed for use in the lumbar spine, with the first human implantation occurring in the early 1990s. The cylindrical titanium cages were threaded to screw into the endplates, thereby stabilizing the device and allowing for increased fusion rate with a stand-alone anterior device.
Ray and colleagues developed a similar titanium interbody fusion device which was initially used in posterior lumbar interbody fusions (PLIF), but expanded to include ALIF procedures (anterior lumbar interbody fusions). In 1985, Otero-Vich reported using threaded bone dowels for anterior cervical arthrodesis, and femoral ring allograft bone has subsequently been fashioned into cylindrical threaded dowels for lumbar application.
Currently, there are a wide number of available interbody fusion devices of varying design and material, including:                1) Cylindrical threaded titanium interbody cages;        2) Cylindrical threaded cortical bone dowels; and        3) Vertical interbody rings, boxes and wedges.        
A typical intervertebral fusion cage is a large, hollow cylinder made of some type of metal, usually titanium. It is designed as a “cage” so that bone graft can be placed inside the hollow cylinder. Holes throughout the cage allow bone to form around and through the cage to allow a spinal fusion to occur between two vertebrae. Many of the newer types of intervertebral fusion cages are also designed to facilitate an open incision or a laparoscopic procedure.
An intervertebral fusion cage serves a couple important purposes. First, it distracts the vertebrae, making more room for the nerves, thereby decreasing pinching and irritation. The strong ligaments that surround the disc are also tightened, which decreases the segmental instability between the two vertebrae and decreases the mechanical pain in the spine. The cage also holds the two vertebrae in the correct position until a fusion occurs.
There are several drawbacks with existing approaches and techniques, such that further research and improved designs are desirable. Increased morbidity of anterior in-situ cage placement is not justified when less anatomic correction of the disc space is possible. Additionally, current PLIF and transverse lumbar interbody fusions (TLIF) cage and allograft placements require large dissections for exposure. PLIF and TLIF approaches also weaken existing posterior elements via bony destruction resulting from the operative procedure used to access the disc space.